1. Field of the Invention
The present invention relates to a focus detecting device, and more particularly to a focus detecting device for a single-lens reflex type camera for detecting a focusing state of an object lens by sensing an image formed through the object lens.
2. Description of the Prior Art
A focus detecting device of this type has been well known in the art and various methods for detecting a focusing state have been proposed. For example, Japanese Patent Application Laid-Open No. 52-95221 (laid open on Aug. 10, 1977) discloses a method for detecting the focusing state of the object lens based on a relative positional relationship of a plurality of images formed by an optical system by light fluxes transmitted through different areas of a pupil of the object lens by arranging the optical system and sensors for sensing the images behind an anticipated focal plane of the object lens.
The focus detecting device which adopts the above detection method has advantages in that the optical system for forming the images is relatively simple and not only the focusing state of the object lens but also a defocus amount can be detected. On the other hand, since the optical system for forming the images and the sensors for sensing the images must be arranged behind the anticipated focal plane of the object lens, a total length of the device is long. Accordingly, when the focusing state detection device which adopts this detection method is incorporated in a camera to detect the focusing state of the imaging lens, it is difficult to properly mount the optical system and the sensors in the camera body.
A prior art camera having such a focus detecting device is explained below. FIG. 1(a) shows the optical system and the sensors arranged around a pentaprism, which is disclosed in Japanese Utility Model Application Laid-Open No. 57-40919 (laid open on Mar. 5, 1982). Two light fluxes Pa and Pb emitted from a point Q on an object impinge on different areas 101a and 101b of a pupil of an imaging lens 101 and form a primary image Q1 on a focusing screen 103 through a mirror 102. The two light fluxes Pa and Pb transmitted from the image Q1 through the focusing screen 103 pass through a condenser lens 104 and are reflected by a half-mirror 105a of a beam splitter 105, and pass through a spectrum sensitivity correction filter 106 for a detected light and a light distribution means 107 and directed to two secondary focusing lenses 108a and 108b, are reflected by reflection mirrors 109a and 109b and form separate secondary images on detection planes of sensor arrays 110a and 110b. The object light passed upward through the half-mirror 105a is directed to an eye lens 112 through a pentaprism so that the object image can be viewed.
Referring to FIG. 1(b) in which the above example is viewed from the eye lens 112, a detection unit holder 114 is threadedly mounted on a finder 113 to which the focusing screen 103, the condenser lens 104 and the pentaprism 111 are mounted in union. Mirror supports 114a and 114b are formed in union at left and right ends of the unit holder 114, and the reflection mirrors 109a and 109b are bonded to the mirror supports 114a and 114b. A detection optical system unit 115 is attached to the unit holder 114 by a screw 116. The correction filter 106 is bonded to an opening 117 of the unit 115 facing the beam splitter 105. A spacer ring 118 is provided in the unit 115 to hold the secondary focusing lenses 108a and 108b at predetermined positions, and a holding member 119 which is integral with the light distribution means 107 is fixed to the unit 115. Those are located below the eye lens 112 and the two light fluxes Pa and Pb deflected by the half-mirror 105a are directed from the reflection mirrors 109a and 109b to the sensor arrays 110a and 110b located above the eye lens 112. The unit holder 114 and the mirror supports 114a and 114b, the unit holder 114 and the unit 115, the unit 115 and the spacer ring 118, and the spacer ring 118 and the holding member 119 can be relatively position-adjusted.
In this example, since the detection optical system and the sensors are separately supported, the positional relationship of the detection optical system and the sensors varies when the position of the detection optical system is adjusted relative to the imaging lens 101. Accordingly, a number of steps are required to adjust the entire device.
FIG. 2(a) shows another example of the prior art device in which the detection optical system and the sensor are arranged on the bottom of the camera body, which is disclosed in Japanese Patent Application Laid-Open No. 58-54323 (laid open on Mar. 31, 1983). A light flux transmitted through an imaging lens 201 passes through a main mirror 202 and is reflected by a sub-mirror 203 forward-diagonally and downward of the camera body, passes through a field lens 204 and is directed to a reflection mirror 205. At an area A shown in FIG. 2(c), the light flux is reflected backward by the reflection mirror 205 and directed to a beam splitter 206 and split to left and right by reflection planes 206a and 206b of the beam splitter 206 [see FIG. 2(b)]. The light reflected right-diagonally and forwardly by the reflection plane 206a passes through a secondary focusing lens 207, is reflected backward by the reflection mirror 208, is reflected leftward by a reflection mirror 208 and reflected downward by a reflection mirror 210 and focused on a detection plane of a sensor array 211 located at an area B shown in FIG. 2(c). On the other hand, the light reflected leftward by the reflection plane 206b is reflected forward by a prism 212, passes through a secondary focusing lens 213, is reflected rightward by a prism 214, is again reflected downward by a prism 215 and is focused on a detection plane of a sensor array 216.
In this example, the light fluxes from the periphery of the exit plane of the detection optical system, the light fluxes transmitted through the external of the lens of the optical system or the light fluxes reflected by the lens surface may impinge to the sensor as stray light.
In order to incorporate such a focus detecting device in a camera, it is necessary to position the detection optical system and the sensor in the camera body with a predetermined relation to the imaging lens and hence it is necessary to adjustably support the optical system and the sensor in the camera body. The support method is disclosed in Japanese Utility Model Application Laid-Open No. 57-8816 (laid open on May 31, 1982), No. 57-187416 (laid open on Nov. 27, 1982) and Japanese Patent Application Laid-Open No. 58-78128 (laid open on May 11, 1983), in which a sensor block including a sensor or an optical system and the sensor are supported by three or four bolts and the sensor block is urged to the bolts by a spring so that the position of the sensor block is adjusted by the bolts. In this method, however, it is difficult to independently adjust the position of the sensor block in orthogonal X - Y and Z directions and a long time is required for the adjustment.
When such a focus detecting device is incorporated in the camera, it is necessary to pay attention to a relationship between an incident plane of the detection optical system and an imaging plane to prevent a light other than an imaging light from reaching the imaging plane (film plane).